Extended Forecast Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Thu Jul 6 2023 Valid 12Z Sun Jul 9 2023 - 12Z Thu Jul 13 2023 ...Heat expected to rebuild from the Southwest to the central Gulf Coast this weekend into next week... ...Heavy rain/runoff concerns likely to focus over portions of the south-central Mississippi/Tennessee Valley region and Mid-Atlantic/Northeast Sunday into Monday... ...Overview... Most guidance shows the large scale flow settling into a fairly persistent pattern from late weekend through next week. This configuration will consist of an eastern U.S. mean trough anchored by a deep upper low tracking from the western shore of Hudson Bay into northern Ontario, a strengthening Rockies/High Plains upper high that should drift a bit westward to around the Arizona/New Mexico border along with a ridge extending to its north-northwest, and mean troughing offshore the Pacific Northwest. Shortwaves within the upper trough east of the Rockies and associated waves/fronts will likely provide a focus for multiple episodes of showers and thunderstorms with areas of heavy rainfall possible. Meanwhile, the upper ridge will support an expanding area of hazardous heat between the Southwest U.S. and central Gulf Coast region, likely persisting into the week two time frame per Climate Prediction Center forecasts. ...Guidance Evaluation/Predictability Assessment... The model blend for this afternoon's forecast remains consistent with the overnight forecast. A blend of operational deterministic models was used for the beginning of the period, and ensemble means were introduced to the blend for the second half of the period. This blend captures the large scale pattern well, with some uncertainty remaining surrounding smaller scale features. Model solutions do not agree on timing/positions of shortwaves rounding the base of the trough in the East, which will affect surface features and sensible weather. Relative discrepancies of the CMC and UKMET (more and less amplified versus consensus, respectively) cancel out among the minority components of their solutions over the East. At least their QPF patterns over the Northeast early in the period have trended closer to the GFS-GEFS/ECMWF-ECens that have held up well over the past 24 hours aside from a little east-west variability. Farther west there is decent clustering for the northern Ontario upper low, with typical spread in track for forecasts several days out in time. Specifics of position/embedded waves for the leading cold front that drops into the northern tier by Monday and continues south/east thereafter will depend on lower predictability shortwave details so expect some continued variability with that aspect of the forecast. The mean trough off the Pacific Northwest coast should contain multiple embedded features, with the axis of the overall trough likely becoming aligned somewhat farther west late in the period in response to an upper ridge that amplifies near 160W longitude. ...Weather/Hazards Highlights... 18Z Update: Minor changes were made to the Day 4 Excessive Rainfall Outlook. The Slight Risk area over the South was expanded a bit west into central Oklahoma where heavy rainfall in the short term period could increase the sensitivity to flooding. The Slight Risk area in the South was also expanded northeast into central Tennessee where forecast rainfall has increased slightly. Another minor change was made to the Slight Risk area in the Northeast, expanding it southwards to include parts of southern MD and northern VA. It is possible this area may need to be pulled even farther south into VA in future updates, but confidence was not high enough to expand it further at this time. For the Day 5 Excessive Rainfall Outlook, the Slight Risk area in the South was shifted southeast and expanded slightly to account for latest model trends in precipitation. The Slight Risk area now includes eastern Louisiana, southern Mississippi, southern Alabama, and part of the Florida Panhandle. During the Days 4-5 period covering Sunday through Monday night, there will be two areas of greatest focus for heavy rainfall potential. One should extend from around Oklahoma eastward into the Tennessee Valley and southeast toward the east-central Gulf Coast, which will likely be the continuation of a multi-day period of convection starting out over the central Plains in the shorter term. Guidance has on average shifted southward with this area of rainfall, and this is reflected in the updated EROs. By Day 5 there are signals for a possible band of convection extending back northwest into the central Plains within an axis of favorable moisture and instability, to the north of a front that begins lifting north as a warm front. However, guidance is sufficiently divergent to recommend only a Marginal Risk over that area for now. The other significant region of focus will be the northern Mid-Atlantic into the Northeast on Day 4, with good continuity for the Slight Risk area from recent outlooks. Most guidance suggests anomalous moisture will persist over the Northeast into Day 5 but the full range of guidance provides mixed messages for rainfall specifics, favoring a Marginal Risk as a starting point. Continue to expect some bands of rainfall across the far northern tier with an initial weakening front and a stronger front pushing south from Canada. Some east-west training of this activity could be possible but relatively modest moisture anomalies and moderately dry antecedent conditions still favor no risk area for now. By Tuesday-Thursday the East should see at least a brief drier trend while more showers/thunderstorms develop over parts of the central U.S. into the Mississippi Valley/Great Lakes as the front reaching the northern tier early in the week continues pushing south and east. Some of this rainfall could be locally heavy, with location and magnitude details gradually becoming better defined over the coming days. Most of the West should remain dry through the period. Strengthening of the initial southern Rockies/High Plains ridge and a slight westward drift with time will likely expand coverage of hazardous heat across the southern tier during the forecast period. Expect the greatest high temperature anomalies of 10-15F above average to be near the upper high center (western Texas and parts of New Mexico) with a broadening area of highs at least a few degrees above average elsewhere, including the Southwest by the start of next week. Likewise, expect more coverage of maximum heat index values reaching 110F or greater over southern/eastern Texas into the Lower Mississippi Valley, while lower humidity will lead to maximum heat index values over the Southwest near or slightly below the expected air temperatures reaching up to 110-115F or so from late weekend onward. Low temperatures in the upper 70s to low 80s will not provide much relief overnight and could exacerbate heat stress as well as yield some daily records for warm minimum temperatures. There may be a few daily records for high temperatures as well. Climate Prediction Center forecasts indicate this heat should extend beyond next Thursday. The central Plains should see temperatures rebound to near normal for a couple days or so after a cool Sunday (highs around 5-10F below average). A cold front dropping south from Canada should bring temperatures back down to 5-10F below average over the northern into central Plains by next Wednesday-Thursday. Parts of the Interior Northwest/far northern Rockies could also see highs up to 10-15F above average Sunday-Monday, then a moderating trend is expected mid-week. Much of the eastern half of the lower 48 should see near to somewhat below average temperatures Sunday-Monday and more near-average temperatures thereafter. Dolan/Rausch Hazards: - Heavy rain across portions of Arkansas and the Tennessee Valley, Sun, Jul 9. - Heavy rain across portions of the Deep South and the Lower Mississippi Valley, Mon-Tue, Jul 10-Jul 11. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jul 9-Jul 10. - Flooding possible across portions of the Central Plains. - Flooding likely across portions of the Northern Plains. - Hazardous heat across portions of the Pacific Northwest, Sun, Jul 9. - Hazardous heat across portions of the Southern Plains, the Lower Mississippi Valley, California, and the Southwest, Sun-Thu, Jul 9-Jul 13. - Heavy rain across portions of western mainland Alaska, Sun, Jul 9. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw